Why can not use reference in operator function?

[Parmenides <mobile dot parmenides at gmail dot com>]

Hi,

I have the following code:

#include <iostream>
#include <cstring>
using namespace std;

class Int{
int x;

public:
Int(int i = 0)
{
x = i;
}

~Int()
{
}

void show()
{
cout << x << endl;
}

friend Int operator+(Int &a, Int &b)  // Because 'b' is a reference to
object rather than an object
{
return Int(a.x+b.x);
}
};

int main()
{
Int i(3), j;
j = i + Int(6);   // This can not call constructor.
j.show();

return 0;
}

Gcc issues error message, but VC++ 2010 compile it successfully. I
tried to modify

                friend Int operator+(Int &a, Int &b)

to

                friend Int operator+(Int a, Int b)

both compiler can get it pass. Why Gcc does not want to convert a
'int' to a 'Int' object in the  '+' operator function when its second
parameter is a reference to object rather than an objetc?

I further modified

                j = i + Int(6);

to

               j = i + 6;

both compiler can get it pass again. Therefore, I think it seem that
Gcc encourage programmers to use implict conversion like 'i + 6'
rather that explict conversion like 'i + Int(6)'. Is this right? If
so, does this practice comfore to the C++ standard?